Modelling a 36kV utility-scale photovoltaic plant according to the italian transmission system operator grid code

The Italian and European electricity generation mix is becoming increasingly dominated by renewable sources, driven by the need to reduce greenhouse gas emissions and pollutant levels. Photovoltaic generation is the leading technology in this transition: the maturity of the technology and the relocation of manufacturing outside Europe have lowered its cost to the point that plant sizes have grown steadily, from single-user installations to modern utility-scale photovoltaic fields. This growth, however, is not free of drawbacks. The variability and uncertainty of the solar resource introduce several critical issues for the management of the transmission network, and these issues become more severe as the installed capacity increases, since inverter-based generation behaves differently from conventional synchronous power plants. In recent years, the Italian Transmission System Operator has issued several updates to its Grid Code, together with a set of annexes dedicated to renewable sources, covering connection schemes and the tests required before a plant may be authorised to inject power into the network. Within this regulatory framework, the present thesis, developed during an internship at Gridshape, an electrical design company based in Padua, focuses on the development of a DIgSILENT PowerFactory® model — the software accepted by the TSO — of a case-study photovoltaic plant adopting the most recent connection concept defined by the TSO, namely the 36 kV scheme, so that all the requirements set out in the annexes can be studied and described. The main reference is Annex A.68, which governs the connection of photovoltaic plants. The model makes it possible to derive the plant capability curves, both PQ and VQ, and to show through them how sensitive these curves are to variations in the internal topology, as well as how a plant can be modified when the requirements are not met. The analysis then moves to primary and secondary power regulation, and to the settings required on the Power Plant Controller to regulate active and reactive power under the different scenarios defined by the Grid Code. These scenarios are also reproduced in the test configurations prescribed by Annex A.18, which offer a clearer picture of plant behaviour under standard conditions. Finally, the use of a commercial PPC, permits to the investigate some non-standard conditions of both the grid and the devices involved, in order to assess how the plant responds to situations that are not foreseen by the regulation but are closer to actual operating conditions.